The mitochondrial diseases are a heterogeneous group of disorders that have been defined by deficits of the Mitochondrial respiratory chain. These diseases are clinically and biochemically diverse. Mitochondrial dysfunction is a relatively common cause of disease in children, leading to a wide variety of pediatric problems, including developmental delays, hypotonia, sensorimotor impairment, seizures, and organ failure. There is also significant morbidity and mortality associated with mitochondrial diseases in children. The identification of pathogenic mutations in mitochondrial DNA has resulted in a genetic classification of mitochondrial diseases. A number of myopathies and encephalonryopathies result from mutations in mitochondrial DNA-encoded tRNA genes. Investigations are being conducted to understand the molecular basis for the biochemical alterations of mitochondria associated with these mitochondrial DNA mutations. While the genetic identification of mitochondrial DNA mutations will aid the genetic counseling of patients, the prognosis of patients is not good. Currently, there are-no reliable treatments or therapies available for respiratory chain deficiencies. The goal of this proposal is to develop strategies to correct respiratory chain deficiencies resulting from mutations in mitochondrial tRNA genes. It should be possible to complement defects associated with human mitochondrial tRNA gene mutations by importing a functional tRNA from the cytosol. The specific goal is to develop a system for the import of tRNA into human mitochondria. Imported tRNAs are expected to complement the defects in mitochondrial translation due to mutations of the human mitochondrial tRNA genes. This investigation will serve as a model for the eventual treatment of human encephalomyopathies caused by mutations in mitochondrial DNA genes encoding tRNAs.